Hitherto, there has been known current diffusion bonding whereby to carry out bonding by heating metal members within a solid phase temperature range to make use of the diffusion phenomenon of atoms. The current diffusion bonding is ideally suited for bonding the same type of materials or different types of materials composed of difficult-to-weld metal materials.
In the current diffusion bonding, members to be bonded, which are held between electrodes and the bonding surfaces of which are under a pressure applied by a pressurizing mechanism through the electrodes, are heated by passing a pulse current between the electrodes. Then, within a solid phase temperature range of the melting temperature or less of the members to be bonded, the bonding is implemented by the close contact of the bonding surfaces resulting from the softening and deformation of the material and the solid phase diffusion phenomenon.
If any slight gap occurs between the bonding surfaces, then defective bonding results. To prevent such defective bonding, the members to be bonded are heated while being subjected to a high pressure applied by a pressurizing mechanism. However, when the temperature of a base material exceeds a softening start temperature thereof, the members to be bonded develop deformation, such as distortion, bend or compression due to the applied pressure.
As a solution, Patent Document 1 discloses a pulse current bonding apparatus which is provided with a pressure sensor that detects a pressure applied to members to be bonded and which carries out feedback control of a pressing force, which is determined by the materials and dimensions of the members to be bonded, on the basis of a detection value of the pressure sensor. More specifically, in order to prevent the pressure applied to the members to be bonded from increasing due to the thermal expansion of the members to be bonded, the pressing force is feedback-controlled such that the detection value of the pressure sensor remains constant after the passage of a current is started (refer specifically to paragraph 0017 of Patent Document 1). The pressing force is controlled by controlling an electric motor that drives a ball screw unit of a pressing device.
Further, Patent Document 2 discloses a current bonding apparatus which is provided with a temperature detecting means that detects the temperature of a bond portion and a member displacement monitoring unit that monitors members to be bonded for displacement. In a state wherein a temporal change in the temperature of the bond portion is positive, if it is detected that the displacement of the members to be bonded is negative, then the pressurizing force applied to the members to be bonded is decreased. The pressurizing is carried out by a generally used mechanism of a hydraulic type, pneumatic type, mechanical type or the like (refer to paragraph 0021 of Patent Document 2).